8.5. Highly Destructive Payload

The most dangerous type of computer viruses will intentionally harm your data or even the hardware in your system. Examples of this type are discussed in this section.

8.5.1. Viruses That Overwrite Data

Many extremely harmful computer viruses simply format the hard drive or overwrite the disk. The Michelangelo virus was among the most infamous of this kind, and the media overreacted to this news. Michelangelo did not attempt to overwrite the entire disk, only parts of the partition (the first 256 cylinders) that system had been booted from. For this reason, it was often feasible to recover data from attacked systems easily.

Over the years, most computer virus researchers have become data recovery experts because of the various kinds of corruption such viruses can cause to a system. Some viruses simply killed the Master Boot Record (MBR) of the system to make them unbootable. This effect was very simple to fix, and Norton Disk Doctor did a particularly good job on its own in many cases. Many data recovery companies charged data restoration by the amount of KBs restored from the hard disk. While it was certainly worthwhile to pay this fee in the case of a real hardware error, it was not cheap to pay for a virus-damaged disk with a corrupted MBR (a single sector).

The Hungarian Filler's damage routine is worth mentioning here. Not only did the virus delete the content of the FAT sectors on DOS, it also filled those sectors with 01 () characters, drawing eight similar, large smiley faces in each FAT sector. Thus when you checked to see what happened to the corrupted disk using a disk editor such as Norton DiskEdit, you would find drawings similar to the one shown in Figure 8.6.

Figure 8.6. The graffito of the Filler virus in a FAT sector.

The activation routine of the virus was encrypted in the virus body and has never been documented in international virus descriptions because of this. Instead, the original Hungarian name of the virus, Töltögetö, was translated into English as Filler.

Some other viruses simply delete files. For example, the AntiPascal family targets Pascal programs and deletes them from the disk. Other variants of the AntiPascal virus create temporary files with hidden, system, read-only flags to fill the disk with data and make the system unbootable by manipulating the master boot record.

Unfortunately, some of the successful computer worms such as W32/Witty also used this method to quickly corrupt the content of disks on the attacked hostswithin minutes.

8.5.2. Data Diddlers

The term data diddler was coined by Yisrael Radai. Fred Cohen also used the term to describe viruses that do not destroy data all of a sudden in a very evident form. Instead, data diddlers slowly manipulate the data, such as the content of the hard disk. This kind of corruption is very dangerous because it can easily end up in backups before anyone notices it. Because backups are often reused, all possible data might be corrupt when the virus attack is finally noticed.

Among the first viruses that caused major damage was Dark_Avenger.1800.A, commonly referred to by the nickname Eddie. The virus was so dangerous that the first company I visited in Hungary as a young researcher to collect the virus sample did not even want to talk about it anymore. They simply nailed the remaining infected diskette to the wall as a bad memory of a computer virus attack.

Apparently, Eddie got its name from a text that is not only stored in the virus but is randomly written all around on the hard disk, except on the FAT, causing the slow death of the system. Thus the virus intentionally avoids destroying the FAT so the system will die more slowly and the virus can happily replicate to new systems during that time.

The virus wrote, "Eddie lives…somewhere in time," on randomly selected sectors of the hard disk. This text could be detected later, but too many files (including databases) would contain it by the time the virus was finally discovered.

Another good example of a data diddler virus is Ripper (which gets its name from Jack the Ripper). Ripper exchanges two data word values in a randomly selected sector and writes back the result to the disk. This is a serious corruption! Furthermore, we have noticed that in some rare cases, such a data-diddler effect can be responsible for minor mutations of computer viruses on Ripper-infected systems. In most cases, a binary virus will be destroyed with this kind of random data manipulation, but in theory such a corruption can lead to a working variant, a tiny mutation that might not be detected by antivirus products using the same definition as they used to detect the original variant. The Russian virus, WordSwap, uses a similar approach and swaps two textual words when files are written to disk.

8.5.3. Viruses That Encrypt Data: The "Good," the Bad, and the Ugly

The Disk Killer virus was among the firsts to utilized encryption to attack your data. Disk Killer is a boot virus and was first reported in the United States in June of 1989. The virus spread wildly in Europe a few months later. In about 48 hours after the infected system is booted, the virus calls its payload, which displays a message and scrambles the content of the hard disk using a simple XOR encryption, starting at the partition table. As a result, the virus makes the system unbootable. This is what an attacked system would see on screen:

Disk Killer Version 1.00 by COMPUTER OGRE 04/01/1989

Warning !!

Don't turn off the power or remove the diskette while Disk Killer is Processing!

PROCESSING

After the virus has finished running the encryption it displays this message:

Now you can turn off the power.

I wish you luck !

The encryption in the virus was weak, and therefore restoration of the disk was possible using special decryption tools4. The routine, however, contained some minor errors in the cipher code, which made recovery impossible in some cases.

Another boot virus, KOH, was developed in 1993. It uses the IDEA cipher to encrypt the disk and asks the user to type in a password. Although KOH encrypts the disk similarly to Disk Killer, its goal is not to keep users' data hostage or to damage it, but to protect users' data. This is exactly why this virus generated attention and has been discussed as a so-called "good virus." Mark Ludwig popularized the idea in his book in 19955. Unfortunately, he also made this virus and many others available. Not surprisingly, dozens of variants of the KOH virus exist today6.

Other viruses, such as the Slovakian One_Half that appeared in 1994, use a different technique. One_Half slowly encrypts the disk with a simple encryption method. As long as the virus is active in memory, the disk is accessible to the user because the virus decrypts the encrypted sectors when they are accessed. Finally, after the virus has encrypted half of the disk, it displays the following message:

Dis is one half.

Press any key to continue…

One_Half's disk encryption is a kind of forced symbiosis between the system and the virus. The virus author did not want his creation to be easily removed from the disk. If someone attempts an incompetent manual repair, such as replacing the infected MBR with a clean one, the disk will remain encrypted, and the user will most likely lose his or her data. Several antivirus vendors argued that repairing virus code is sufficient, and such data corruptions should not be the focus of antivirus software. Users saw things differently, however, and antivirus software followed the practice of SAC's (Slovakian Antivirus Center's) repair tool, which decrypted the content of the encrypted sections of the disk and removed the virus from the MBR and the files at the same time.

Similar attacks were also seen on Win32, although with less success. Inspired by the One_Half virus, in December of 1999, the W32/Crypto virus attempted to encrypt the content of the DLLs on the system using the Microsoft Crypto API and strong encryption. This virus, however, contained too many Windows version dependencies and other minor bugs, which caused unwanted system crashes.

Of course, Trojan horse programs such as the AIDS Information Diskette in 1989 already had been used to scramble users' data (as discussed in Chapter 2, "The Fascination of Malicious Code Analysis"), in an attempt to collect a bounty from the users of compromised systems. It was expected that similar attacks would be executed by using an asymmetric cipher. This attack was discussed in a book recently as a "crypto" virus. In fact, the authors of the book claim that they had created such a virus on the Mac in 1996, but "it was a top priority" for them not to release the virus7.

Such crypto viruses use the public key of the attacker to encrypt your data without giving you a chance to decrypt it. Indeed, you would need to pay the bounty to restore your data. In contrast, a simple symmetric cipher easily allows the recovery of the data on the attacked systems because the encryption algorithm (and typically the keys themselves) are stored in the virus, and thus, they can be extracted and used directly to decrypt the data without involving additional secret information held by the attacker.

8.5.4. Hardware Destroyers

Depending on the chipset of the computer and its actual vendor, its Flash BIOS can be updated using software. Nowadays, most PCs use Flash BIOS to update code quickly that would normally be burnt into chips with no chance of a fix. In the early '90s, virus researchers predicted that Flash BIOS would be attacked by computer viruses.

The infamous Taiwanese virus, W95/CIH, successfully killed at least an estimated 10,000 PCs in 1998 by overwriting the boot strap portion of Flash BIOS code. The virus used I/O port commands in kernel mode to access the Flash BIOS. Such port commands could also be executed in user mode, but doing so would have allowed easy protection against the virus's activation routine, and the author of the virus obviously wanted to avoid that.

For instance, a kernel-mode driver could have been loaded on the system to hook the I/O ports that the virus used as an "Open Sesame!" sequence to access the Flash BIOS for write. A VxD could easily capture the request of user-mode code in this way, protecting the machine against such an attack. However, CIH executes the I/O port commands in kernel mode, which another VxD cannot capture and block.

Obviously, a more challenging attack would have been Flash BIOS infection. In fact, such a proof-of-concept virus exists, written by the virus writer Qark in 1994. Fortunately, however, such a virus is fairly BIOS codespecific and hard to implement in a generic way that would support several systems.

Simpler activation routines involve setting a random CMOS (complimentary metal oxide semiconductor) stored boot-up password for the PC. Viruses such as the AntiCMOS family use this trick. The virus takes advantage of the boot protection password, which the user might not be able to change without first uncovering the machine and draining the CMOS battery or, alternatively, clear the content of the CMOS using a jumper setting on newer motherboards.